(1) Field of the Invention
The present invention relates to high strength fiber of polytetrafluoroethylene (called PTFE hereinafter) having a strength of at least 0.5 GPa, and a method for manufacturing the same, further, ultra high strength fiber of PTFE having a strength of at least 1.0 GPa, and a method for manufacturing the same.
(2) Description of the Prior Art
PTFE is one of fluorine resins, and FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkoxy Group copolymer), and ETFE (tetrafluoroethylene-ethylene copolymer) are included in the fluorine resins.
Each of the above described fluorine resins has superior heat resistance, chemical resistance, water and moisture resistance, electric insulating property, and incomparable non-adhesiveness and surface wear resistance. Among the above fluorine resins, PTFE has most preferable heat resistance, chemical resistance, and water and moisture resistance. Accordingly, PTFE fiber also has the same preferable feature as the above described feature of PTFE resin itself. PTFE fiber is manufactured and sold by American Du Pont Co. and Japanese Toray Fine Chemicals Co. Details of their methods for manufacturing PTFE fiber are not known, but characteristics of PTFE fiber manufactured by each of the above companies does not have significant difference mutually.
Smith et al. (U.S. Pat. No. 2,776,465) disclosed highly oriented shaped tetrafluoroethylene article and process for producing the article. Smith et al taught PTFE fiber obtained by drawing a PTFE monofilament formed by paste extrusion after heat treatment at a temperature higher than crystal melting point of PTFE. As far as the above steps of operation, the disclosure by Smith et al is identical with the present invention. However, Smith et al did not teach any of the free end anneal (FEA) of PTFE monofilament, which is the key operation of the present invention. Accordingly, strength of the PTFE fiber obtained by the Smith et al's disclosed process is as low as approximately 2.4 g/d (0.45 GPa) (Example IX).
Katayama (U.S. Pat. No. 5,061,561) disclosed yarn articles comprising a tetrafluoroethylene polymer and a process for producing the article. Katayama taught a PTFE fiber having a tensile strength in a range 4-8 g/d (0.74-1.49 GPa) (col.5, lines 28-32). However, the PTFE fiber is obtained by drawing porous PTFE material comprising nodes connected by fibrils as a starting material at a temperature higher than melting point of PTFE crystal. Therefore, the PTFE fiber by Katayama is obtained by an entirely different process from the present invention.
The porous PTFE material, the raw material, is obtained by the process described in col. 5, line 65 col. 6, line 8 in the reference (U.S. Pat. No. 5,061,561). The porous PTFE material itself is expensive, and PTFE fiber obtained by manufacturing of the porous PTFE material is naturally more expensive.
Generally speaking, a mechanical strength of PTFE fiber is rather at a lower level as fiber than the maximum level. Among various fibers of fluorine resins, the mechanical strength (GPa) of PTFE fiber is approximately 0.16, and is slightly larger than those of FEP (0.04) and PFA (0.07) but inferior to that of ETFE (0.25).
Comparing with general fibers made from materials other than fluorine resins, difference in the mechanical strength is significant, for instance, such as high strength string of nylon (0.7), high strength string of polypropylene (0.66), and high strength string of polyester (0.55).
The fact that the mechanical strength of PTFE fiber is far inferior to that of other general fiber is considered to be one of the serious problems which prohibits PTFE fiber from being used in wider utilizing fields in consideration of the most preferable feature such as aforementioned heat resistance, chemical resistance, and water and moisture resistance.
Further, currently, high strength fibers or ultra high strength fibers made from various materials which are extending gradually a variety of kinds have been developed. Although there are other terms such as high elastic or ultra high elastic fibers, these fibers are almost similar with the above high strength or ultra high strength fibers. Therefore, only the high strength or ultra high strength fiber is restrictively used in this specification as for the term including the high elastic or ultra high elastic fiber.
General definition for the high strength or ultra high strength is not established. However, in this specification, a fiber which can guarantee a mechanical strength of approximately 0.5 GPa is called the high strength fiber, and a fiber which can guarantee a mechanical strength of at least 1 GPa is called the ultra high strength fiber.
Considering raw materials for the high strength or ultra high strength fiber by dividing conventionally the raw materials into two categories such as a bending chain polymer and a rigid linear chain polymer, only three polymers such as polyethylene of the bending chain polymer, and aramid and polyallylate of the rigid linear chain polymer are considered to be suitable for the raw materials, and further, if the raw materials are restricted to polymers for general use, only polyethylene is considered to be appropriate.
As commercial products, "Kevlar" (made by E. I. du Pont de Nemours & Co.) and "Technola" (made by Teijin Co.) of aramid group, "Vectran" (made by Kurare Co.) of polyallylate, and "Dynima" (made by Toyobo Co,), "Techmiron" (made by Mitsui Sekiyu Chemical Co.), and "Spectra" (made by Allied Chemical Corp.) of polyethylene group are available.
The above mentioned commercially available (ultra) high strength fibers have the following problems. First, polyethylene (ultra) high strength fiber has poor heat resistance. On the contrary, (ultra) high strength fibers of aramid and polyallylate are superior to polyethylene in heat resistance, but are generally inferior in water resistance which is very important in practical use, especially in hot water resistance, as a common defect of polymers obtained by a condensation polymerization.
Further, as for a common problem for all of the (ultra) high strength fibers, expensiveness is pointed out. The reason of expensiveness can be considered as a cost-up caused by, in cases of aramid and polyallylate, their very special raw material monomers which necessitate to be synthesized especially, and in case of polyethylene, an expensive new investment in manufacturing facility and a problem such as a slow speed of production. In consideration of the above problems, invention of an (ultra) high strength fiber, which has no aforementioned serious problems and can be manufactured from conventional monomers by a relatively simple process, has been expected from commercial markets.